1. Technical Field
This invention relates to battery monitoring devices, and more particularly to such devices for providing fast recharging of nickel-cadmium batteries and including controls therein to avoid overcharging such batteries.
2. Description of the Prior Art
The use of rechargeable nickel-cadmium (NiCad) batteries for consumer products is well established. Such rechargeable batteries are frequently used in communication devices such as cellular and cordless telephones and other mobile telecommunications products. Unlike disposable batteries, however, the NiCad batteries require recharging upon dissipation of the electrical energy stored therein. The recharging period of the NiCad batteries, if too long, may thus diminish the effectiveness of the device which incorporate the batteries. There have thus been prior art attempts to speed up the charge rate in order more quickly to restore the batteries to full capacity.
The normal recommended continuous charge rate for NiCad batteries is a standard charge rate which provides a 10 to 15 hour charge time at a C/6.5 to C/10 charge current, where C is the capacity of the battery in millamps-Hours. A Trickle Charge is an infinite charge time at a C/20 to C/30 charge current. A Fast Charge is a 2 to 4 hour charge time at a C/1.3 to C/2.7 charge current. And a Rapid Charge is a 15 to 30 minute charge time a 5 C to 2.5 C charge current. The standard charge rate thus results in a time of 10 hours or more to recharge a battery pack. Such a time requirement is excessive, however. If the batteries powering a device are discharged prior to completion of an operation or task, it is necessary for the user either to wait for a recharge or to replace the battery pack with a fully charged replacement pack. The first approach, as noted above, is typically highly time consumptive while the second is expensive.
Accordingly, the prior art has developed several approaches to reducing the recharge time for rechargeable batteries, including various techniques to avoid overcharging the units.
One approach for permitting the rapid charging of NiCad batteries is to charge the battery with a large amount of current (1 C to 2 C rate) and then employ monitors to measure either the change in voltage or change in temperature characteristics, or both. A fully charged battery that is subjected to a large charging current, however, will show a reduction in terminal voltage over time and and increase in temperature over time. Such detrimental effects are seen when large amounts of charging current are repeatedly used to charge a nearly full to fully charged battery.
In another approach to the problem, battery manufactures have conducted research into battery characteristics under charge and have developed special cells. Thus, some newer cells are characterized by a charge rate of C/3. These cells are capable of withstanding the higher charge rate indefinitely. The time required for fully charging such cells has thus been reduced to approximately four hours.
Although NiCad batteries containing these specially developed cells are capable of being charged at a higher rate than conventional NiCad batteries, batteries containing such cells are expensive. Also the cost of a device which includes sensors and circuitry for measuring battery voltage and temperature characteristics for determining battery capacity is similarly expensive because of these additional components.